Wind energy applications require accurate measurement of wind speed and turbulence. For example, optimizing the design and operation of wind farms requires understanding the complex interaction between atmospheric boundary layer flow and wind turbines. In particular, understanding wind turbine wakes is important for optimizing power output and minimizing fatigue loads on the turbine blades, especially within wind farms.
Wind measurements for wind energy applications have been obtained mainly with cup anemometers. However, cup anemometers cannot measure wind direction and require the erection of tall towers that can disturb wake flow. Therefore, there is a need for remote sensing instruments that can perform wind speed measurements without affecting the flow. Light detection and ranging (lidar) systems are now being deployed to measure the velocity of the inflow and the wake of wind turbines.
A Doppler lidar is a remote sensing instrument that measures wind velocity through the evaluation of the Doppler shift of a laser beam emitted into the atmosphere and backscattered from aerosol particles (e.g., dust, droplets) in the atmosphere. The Doppler lidar measures the velocity component along the line-of-sight of the laser beam, derived from the frequency difference between the emitted and backscattered signals. This is often done coherently, by observing the heterodyne signal between the laser light and what is returned from the aerosol. Velocity measurements at locations along the beam can be performed by simply staring the beam along a fixed direction. Alternatively, a two-dimensional measurement can be performed by raster scanning a highly collimated laser beam over the scene of interest. The range or distance at which the measurement is made can be controlled by focusing the laser beam or from the signal round-trip time-of-flight.
FIG. 1 is a schematic illustration of a wind turbine wake. The far-wake region (at a distance beyond about three rotor diameters (3D) downstream of the wind turbine) is characterized by wake meandering and merging of distinct airflows. In this far-wake region, the maximum wind velocity is about 10 m/s and features are about 1-10 meters in size. RADAR or potentially commercial off-the-shelf (COTS) lidar systems can measure wind velocity in this region. The near-wake region (less than 3D downstream) is characterized by vortices, turbulence, and wake depression. In this near-wake region, the required spatial resolution is about 10 cm to 1 m, with local velocities in excess of 10 m/s, limiting the total measurement time to much less than a second. Downstream distances from 1D to 3D can potentially be measured with existing scanning lidars. However, it is difficult or impossible to measure wake characteristics in the near-rotor region (within one rotor diameter downstream from the wind turbine) using existing lidars. Existing outdoor scanning lidar approaches are too slow to obtain high velocity wind flows in large volumes and typically provide poor range resolution. Wind-tunnel Doppler measurement techniques most often employ continuous wave (CW) lasers to produce sheets of light where scattering is obtained from seeded aerosols. These approaches present problems for wind turbine measurements due to unwanted background signal from the sun, range limitations, slow scanning of the laser sheet and the difficulty of seeding air flow on a large scale. Further, it is desired to obtain three orthogonal components of velocity at each range location to provide a three-dimensional velocity profile of the scene.
Therefore, a need remains for a remote sensing lidar that can measure high speed, vector wind velocities with good range resolution in the wake of a wind turbine.